Effect of Temperature on feasibility Changing Temperature will change the value of – T∆S in the above equation ∆G = ∆H – T∆S If the reaction involves an increase in entropy (∆S is +ve) then increasing Temperature will make it more likely that ∆G is negative and more likely that the reaction occurs e.g. NaCl + aq Na+ (aq) + Cl- (aq) If the reaction involves an decrease in entropy (∆S is – ve) then increasing Temperature will make it more less likely that ∆G is negative and less likely for the reaction to occur. E.g. HCl(g) + NH3 (g) ➝ NH4Cl(s) If the reaction has a ∆S close to zero then temperature will not have a large effect on the feasibility of the reaction as – T∆S will be small and ∆G won’t change much e.g. N2 (g) + O2 (g) 2NO (g) This graph shows how the free-energy change for formation of ammonia varies with temperature above 240 K. ½ N2 (g) + 3 /2 H2 (g) NH3 (g) Applying the equation of a straight line y= mx+c to the ∆G = ∆H – T∆S equation. c = ∆H The gradient of this graph is equal to -∆S The positive gradient means ∆S is negative which corresponds to the equation above showing increasing order. When ∆G <0 then the reaction is spontaneous. In this case at Temperatures below around 460K The slope of the line would change below 240K because ammonia would be a liquid and the entropy change would be different

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Students should be able to:

• use the relationship ∆G = ∆H – T∆S to determine how ∆G varies with temperature

• use the relationship ∆G = ∆H – T∆S to determine the temperature at which a reaction becomes feasible.